In the construction of semiconductor assemblies, semiconductor dies or chips are both electrically and mechanically attached to substrates. In one method of attach, the face of the die containing electrical terminal pads and circuitry, the active face, is bumped with deposits of solder. These solder bumps are aligned and contacted with corresponding terminals on the substrate, the solder is heated to its melting point or “reflow” temperature to form solder joints, enabling mechanical support and electrical interconnections between the semiconductor die and the substrate.
Differences between the coefficient of thermal expansion (CTE) of the die and the substrate often require that the space between the die and the substrate be filled with a reinforcing material, commonly known as underfill, to absorb the stresses created by the CTE differential. Such underfill materials can be applied using at least three different methods.
In the method known as “capillary flow”, the semiconductor die is attached to the substrate through solder interconnections, and then an underfill material is dispensed around the edges of the gap existing between the semiconductor die and the substrate. The underfill is drawn into the gap by capillary action and then cured.
In the method known as “no flow”, the underfill material is dispensed onto a substrate and the semiconductor chip or die is placed onto the substrate. Placement is made such that the solder bumps on the chip are in contact with the corresponding pads on the substrate before the connection by solder reflow. Typically, the underfill is cured during the solder reflow step, though sometimes an additional cure step is required. No flow assembly can also be performed using thermal compression bonding. In this method, the no flow underfill is dispensed on the substrate, the die is placed on the substrate and heat and pressure is applied to the die and/or the substrate to achieve reflow as well as interconnection. As the pressure and heat is applied, the underfill flows out to form fillets and also allows the solder bumps to make interconnection with the pads. The underfill may require an additional cure step.
Capillary and no flow underfill methods are time-consuming due to the fact that they are conducted at the die level. Another major drawback of the no flow system is that if the no flow underfill is a filled system, then filler can interfere with soldering. In addition, the no flow method would require new industry infrastructure to support the thermal compression bonding required, instead of the surface mount technology processes that are standard today.
The method known as “pre-applied” involves applying the underfill onto the active side of a full silicon wafer that has been bumped with solder and singulating the wafer into individual dies at that stage. One key advantage of the “pre-applied” method is the ability to use standard “pick and place” equipment to attach the die to the substrate.
In the main, pre-applied underfills rely on solvent-based adhesive systems in which the solvent must be removed and/or the underfill partially cured to form a solid layer. This process, removing solvent and/or partially curing is known as B-staging. After the underfill is B-staged, the wafer is diced into individual chips. In some operations, a back grinding process to thin the silicon to a controlled thickness may precede the singulation process. The solder balls on the active face are aligned with the terminals on the substrate and the chip is placed on the substrate. The solder is reflowed to form electrical interconnection. If the underfill is not completely cured during solder reflow, a separate underfill cure step may follow.
The current process employed for the assembly of die with wafer level underfill is shown in FIG. 1. Although this wafer level process has some advantages over capillary flow and no-flow, it does have some disadvantages. If the B-stage conditions are not optimized, residual solvent in the wafer level underfill can outgas during reflow, causing voids (which ultimately can lead to failed devices), impeding good solder connections (cold solder joints) or areas that are not contacted by the underfill (non-wets). Further, as the thickness of the underfill layer increases beyond 200 micrometers, removal of solvent from the underfill becomes very difficult. The residual solvent outgases during reflow, causing voids and non-wets. Solvent removal is an additional step in the process and the removed solvent must be disposed of in an environmentally conscious way.
The invention described in provisional U.S. patent application No. 60/638,337 provided a process and composition for a solvent-free wafer level underfill that enables coatings above 200 micrometers and is shown in FIG. 2. The process as described requires the removal of a layer of underfill in order to expose the bumps and enable die attach and electrical interconnection using standard pick and place equipment. Underfill removal can be achieved using a number of methods, including mechanical abrasion or chemical etching. However, the removal of this underfill layer can be problematic. Mechanical grinding has to be fairly aggressive, and a wet process must be used to minimize heat generation that might melt and smear the solder balls. This process step generates contaminated water that must be disposed of, and introduces moisture into a moisture-sensitive process. Chemical etching results in solvent waste, which must be disposed of in an environmentally sensitive fashion.
In some cases the die may be attached to a substrate without removing the layer of underfill over the bumps if the method known as thermal compression bonding is employed. However, this method requires the purchase of special equipment that is not standard within the industry, and its use would reduce the net cost benefits of the pre-applied underfill coating.